David Voas says the secular transition is an ongoing generational replacement of religious people by secular people. People don’t tend to change vis-a-vis religion. Only a tiny percentage of people who are raised secular become religious. People with no religion have great difficulty in acquiring one. Think about a religion not your own such as Hinduism. Here are some Hindu deities and Hindu worship. For most of you, this seems exotic and scary. This is how most secular young people react to religion. You have to be raised with religion to find it natural.
Immigration brings people from more religious countries into secular industrialized nations, but despite this, religion is dramatically in decline in the West.
Modernization has effects. Norway is the most modern country and Niger is the least. The most developed countries are the least religious and the least developed countries are the most developed. Religious decline comes relatively late in the process of modernization.
Most of the world is religious. Yes, because most of the world is not developed. Prosperity brings choice and a reduced willingness to abide by secular authority. Secular and scientific worldviews displace religious worldviews. Mobility brings people into contact with different cultures and beliefs and reduces the hold of traditional ties. Physical security reduces the need for the solace of spirituality.
Religion is a matter of custom and culture. It was the norm at one time. Now secularism is the cultural norm. To the extent that people have contact with religion today it is often in news stories about extremism and abuse. Most Westerners are not rationalists and naturalists, they just have little interest in religion.